Colourless p-phenylene-spaced bis-azoles for luminescent concentrators

Highlights

• New near-UV absorbing fluorophores with bis-azoles backbones were synthesized.

• Dimethylamino imidazole-benzothiazole showed SS > 150 nm and Φ_f > 0.4 in CHCl₃.

• PMMA thin films displayed brilliant green emission and SS > 130 nm.

• The high VIS transmittance and η_opt near 6 support the realization of colourless LSC.

Abstract

We report on the synthesis of new push-pull imidazole-benzothiazole and thiazole-benzothiazole fluorophores obtained in good yields by using direct CH arylation synthetic strategies. Spectroscopic investigations in CHCl₃ solution evidenced that the 1,4-phenylene-spaced imidazole-benzothiazole bearing an electron-donating dimethylamino group achieved a trade-off between fluorescence maximum (516 nm), Stokes shift (165 nm) and a quantum yield higher than 0.4. Dispersions of the selected fluorophore in poly (methyl methacrylate) (PMMA) thin films mostly maintained the optical features in solution with significant light transmittance in the visible region (90% at 440 nm), and a brilliant green emission at 500 nm. Photocurrent experiments performed on PMMA thin films coated over high purity transparent glasses promoted their use in the development of colourless luminescent solar concentrators with optical efficiencies close to 6%.

Introduction

Sunlight concentration is a promising path to cost-effective photovoltaic (PV) technologies. Solar concentration is achieved by collecting the sun radiation incident on a large surface and redirecting it on a smaller area, thus allowing to reduce the amount of photoactive materials, which has the largest impact on the final costs [1], [2], [3]. There are mainly two kinds of solar concentrators, one type is based on geometrical optics [4] and another category resides on luminescent components [3], [5]. Compared to standard concentrators based on geometrical optics, luminescent solar concentrators (LSCs) show several advantages: low weight, high theoretical concentration factors, ability to work well with diffuse light and no needs of sun tracking or cooling apparatuses. LSCs demonstrate an entirely new standard for very large area and highly adoptable solar windows that can translate into improved building efficiency, enhanced UV-barrier layers, and lower cost solar harvesting systems. LSCs consist in a slab of transparent material doped with a fluorophore able to absorb the solar spectrum [6]. The higher refractive index of the host compared to the environment allows to trap a fraction of the emitted photons by means of total internal reflection. Photons are then collected at the edges of the device to produce electric power by means of PV cells. Moreover, the use of commodity plastics such as poly (methyl methacrylate) (PMMA) and polycarbonate (PC) and well consolidated and economic industrial processes for the preparation of LSCs offer encouraging means to include solar energy to the built environment.

However, conventional LSCs are often plagued by a multitude of unfavourable processes that hinder their ability to deliver light to PV cells, in particular fluorescence quenching due to aggregation phenomena between luminescent species [6], [7]. These issues triggered a great flurry of research in the field, leading to a large number of solutions that took into account fluorophore features, polymer hosts and their effective combinations [6]. In the recent years, the research on PV devices based on LSC technology has been focusing on achieving high power conversion efficiencies [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. A simple approach for higher concentrations is to enhance the spectral window of absorption of the LSC, therefore increasing the number of available photons. To this end, multiple dye systems have long been proposed to cope for the narrow absorption characteristic of organic dyes as well as new design solutions [14], [19], [20]. Sloff et al. [18] described a stacked device with a power conversion efficiency of 7.1%, which is, at best of our knowledge, the highest efficiency ever reported for LSC-PV systems. Conversely, a dye mixture in a single slab offers the possibility of cascading of emission via non-radiative processes such as the fluorescence resonance energy transfer (FRET) [19], [21]. Nevertheless, the maximum efficiencies for LSCs were recorded for PMMAs device embedding perylene-based fluorophores [8], [10], [22]. Lumogen F Red 305 is a red-emitting perylene fluorophore, which is considered the state-of-the-art in dyes for LSC applications [6] since it shows a quantum yield (QY) of about 1 even at high concentration (>300 ppm) in polymers [23] and a good photostability [24].

However, the strong absorption in the visible spectrum of perylene-based fluorophores leads to a large degree of colored tinting. In order to overcome this issue, visible-transparent LSCs based on NIR-emissive cyanine salts have been recently proposed as innovative devices with transparency of near 90% in the visible spectrum [12]. This technology is considered very promising and it has received much attention in the scientific community due to LSCs potential application as architectural windows [12], [25], [26].

In connection with these findings, we exploited the potentiality offered by the new 1,4-phenylene-spaced azoles as near-UV absorbing fluorophores characterized by large Stokes shifts (SS) and green emission to be used in colorless LSC with optical efficiencies close to 6. Mixed imidazole-benzimidazole analogues were already proposed by us as fluorophores with SS near 100 nm, very high Φ, and with a bright blue-green emission well retained in the solid state. Notwithstanding their excellent emission features, their fluorescence peaked at λ < 430 nm did not match the working window of a Si-based PV cell (λ ≥ 500 nm) [27], thus impeding their application in LSCs technology [28]. Therefore, new push-pull imidazole-benzothiazole and thiazole-benzothiazole fluorophores, 1a–f and 2a–c (Scheme 1), were efficiently synthesized aimed at retaining the features of imidazole-benzimidazole analogues (3a–c), while shifting their emission to longer wavelengths. Notably, high Φ is still required to maximize the LSC efficiency. Indeed, increasing SS of the fluorophore reduces reabsorption but radiationless internal conversion processes become more probable when the electronic excitation energy decreases, relative to the energy of molecular vibrations [29]. Therefore, an effective trade-off between SS and Φ should be achieved [30], [31].

Thin-film LSC devices were then prepared by dispersing fluorophores in poly(methyl methacrylate) (PMMA) films coated over high optical quality glass slab. 1d and 1f 1,4-phenylene-spaced azoles were selected being their emission higher than 500 nm. The LSCs optical efficiencies were discussed and compared to that measured for LSCs with the same geometry and containing Lumogen F Red 305.